Synthesis, characterization and selective fluorescent zinc(II) sensing property of three Schiff-base compounds

Three Schiff-base compounds, 4-methyl-2,6-bis(1-(2-piperidinoethyl)iminomethyl)-phenol (HL1), 4-methyl-2,6-bis(1-(2-pyrrolidinoethyl)- iminomethyl)-phenol (HL2) and (4-methyl-2,6-bis(1-(2-morpholinoethyl)iminomethyl)-phenol) (HL3), have been synthesized and characterized by elemental analysis, FT-IR, 1H NMR, UV–Vis, electrospray ionisation mass and fluorescence spectroscopy. The emission quantum yield of the compounds increases by ca. 10–17 times by the addition of Zn2+ ion. Introduction of other metal ions of biological and environmental relevance either keeps unaltered or quenches the emission intensity of the ligands. This happens because of large binding constant (∼104 M−1) of the ligand with Zn2+ ion in acetonitrile. Each of the three ligands forms 1: 2 (ligand:metal) complexes which are characterized by single crystal X-ray diffraction analyses. This imposes rigidity to the ligand due to the complexation and, as a result, the radiative decay constant increases and the corresponding nonradiative decay parameter decreases. All of the ligands react with zinc chloride in acetonitrile to form dinuclear complexes which have been characterized by the elemental analysis, FT-IR, UV–Vis, electrospray ionisation mass spectroscopies and single crystal X-ray structural determinations.